Rapid thermal annealing was used to obtain additional information on the phases of the Co-Si system formed during the silicidation reaction between cobalt thin films and silicon substrates. The phase sequence, their stability and their coexistence were investigated by X-ray diffraction and sheet resistance. At an appropriate temperature all three phases are likely to coexist for short times, although the most common observation is the sequential formation of Co2Si, CoSi and CoSi2 as reported earlier. Cursory determination of the activation energy for CoSi2 formation by sheet resistance measurements is 241.1 kJ mol(-1).